Gaseous effluents containing contaminant gases are produced by a variety of operations. For example, sulfur dioxide is generated in various chemical and metallurgical operations, including sulfur-burning sulfuric acid processes, spent sulfuric acid plants, roasting or smelting of sulfidic metal ores and concentrates and the combustion of sulfur-containing fuels (e.g., flue gases from coal-fired power plants). Carbon fuels play a significant role in the generation of electricity, providing energy for heating and fuels for transportation. Most carbon fuels contain sulfur that when burned turns into sulfur dioxide. The sulfur dioxide emitted contributes to a wide range of environmental and health problems. As the emerging economies expand, their demands for energy rapidly increase and as lower sulfur content carbon fuels are depleted, more and more oil and coal reserves having increasingly higher levels of sulfur will be utilized leading to increased sulfur dioxide emissions.
There are also increasing regulatory pressures to reduce sulfur dioxide emissions around the world. The most commonly used method to remove sulfur dioxide is through absorption or adsorption techniques. One approach is to contact sulfur dioxide with an aqueous stream containing an inexpensive base. The sulfur dioxide dissolves in water, forming sulfurous acid (H2SO3) that in turn reacts with the base to form a salt. Common bases are sodium hydroxide, sodium carbonate, and lime (calcium hydroxide, Ca(OH)2). The pH starts at about 9 and is lowered to about 6 after the reaction with sulfur dioxide. A one stage wet scrubbing system usually removes over 95% of the sulfur dioxide. Wet scrubbers and similar dry scrubbing approaches require a capital investment, variable costs due to lime consumption and solids disposal in addition to the energy consumption and utility consumption used to operate the sulfur dioxide removal system.
Another approach is to enhance the sulfur dioxide strength of gaseous effluents in a regenerative process by selectively absorbing the sulfur dioxide in a suitable solvent and subsequently stripping the absorbed sulfur dioxide to produce regenerated solvent and a gas enriched in sulfur dioxide content. A variety of aqueous and organic solvents have been used in regenerative sulfur dioxide absorption/desorption processes. For example, aqueous solutions of alkali metals (e.g., sodium sulfite/bisulfite solution), amines (e.g., alkanolamines, tetrahydroxyethylalkylenediamines, etc.), amine salts, and salts of various organic acids have been used as regenerable sulfur dioxide absorbents. Organic solvents used in sulfur dioxide absorption/desorption processes include dimethyl aniline, tetraethylene glycol dimethyl ether, and dibutyl butyl phosphonate. The capacity of aqueous and organic solvents is diminished by lower pressures and higher temperatures. Accordingly, the sulfur dioxide gas is recovered (and the solvent regenerated) by lowering the pressure and/or increasing the temperature.
U.S. Pat. No. 8,940,258 and US 2012/0107209 A1, describe regenerative sulfur dioxide recovery processes that utilize a buffered aqueous absorption solution comprising certain weak inorganic or organic acids or salts thereof, preferably certain polyprotic carboxylic acids or salts thereof, to selectively absorb sulfur dioxide from effluent gases. The absorbed sulfur dioxide is subsequently stripped to regenerate the absorption solution and produce a gas enriched in sulfur dioxide content.
In these and other regenerative sulfur dioxide recovery processes, there is the potential for accumulation of contaminants in the absorption solution that may interfere with the absorption/stripping operations. These contaminants include divalent sulfur oxyanions, predominantly sulfate salts along with other sulfur-containing species such as thiosulfates and thionates as well as acid gases absorbed from the effluent gas to be treated. The sulfur dioxide-containing effluent gas often contains some sulfur trioxide as well as sulfuric acid mist. In addition, liquid phase oxidation of bisulfite in the absorber leads to the formation of bisulfate. Oxidation tends to be highly temperature dependent and increases sharply as the temperature in the absorber increases. The oxidation process may be catalyzed by the presence of nitric oxide which is often present in the gas to be treated. Iron, sodium, or other metal contamination of the absorption solution may act as an oxidation catalyst and also increase the rate of oxidation of absorbed sulfur dioxide. The addition of a base (e.g., NaOH) restores the buffer capacity of the absorption solution by neutralizing the bisulfate and forming sulfate salts (e.g., Na2SO4) that will accumulate in the recirculating absorption solution and potentially undermine efficient removal and recovery of sulfur dioxide.
As disclosed in U.S. Pat. No. 8,940,258 and US 2012/0107209 A1, sulfate salt contaminant levels in the aqueous absorption solution may be controlled at an acceptable level by periodically diverting at least a portion (e.g., a slip stream) of the absorption solution for treatment to remove sulfate. Treatment comprises evaporating water from the slip stream (e.g., by heating and/or reducing the pressure to flash evaporate water) to produce a concentrated solution supersaturated in the sulfate salt. Sulfate salt crystals are then precipitated from the concentrated aqueous absorption solution in a crystallizer to form a crystallization slurry comprising precipitated sulfate salt crystals and a mother liquor. These publications also describe the addition of an oxidation inhibitor to the absorption solution to reduce oxidation of bisulfite and sulfite to sulfate contaminants.
U.S. Pat. No. 4,122,149 discloses processes for the selective removal of sulfur dioxide from gases using an aqueous absorbent solution. Sulfate and other sulfur oxyanions of heat stable salts that accumulate in the recirculating absorbent solution are removed by contacting it with an anion exchange resin (e.g., a weak base anion exchange resin). Prior to contacting the absorbent solution, the anion exchange resin is converted to the bisulfite form by contact with sulfurous acid. During contact with the absorbent solution, the bisulfite anions are displaced by the heat stable sulfur oxyanions which are thus taken out of the solution. The anion exchange resin is regenerated by contacting it with aqueous ammonium hydroxide to replace the heat stable sulfur oxyanions on the charged resin with hydroxyl anions and thereafter contacting the resin with sulfurous acid to again convert the anion exchange resin to the bisulfite form.
Although the sulfate removal techniques described in U.S. Pat. No. 8,940,258 and US 2012/0107209 A1 can be effective, crystallizer operations, the handling of solids and loss of metal ion from the absorption solution adds to the cost and complexity of the system. Further, the use of anion exchange resins as taught in U.S. Pat. No. 4,122,149, is not applicable to all aqueous absorbent solutions, including those utilizing a polyprotic carboxylic acid salt absorbent, which compete for binding sites on the anion exchange resin and leads to absorbent losses.
A need persists for alternative methods of controlling sulfate contaminants at an acceptable level with minimal capital, energy and operating costs and without significant absorbent loss or complex process steps that would undermine the economic feasibility of the process.